Liquid crystal displays are commonly used as displays for compact electronic apparatuses. Because the liquid crystal in an LCD does not emit any light itself, the liquid crystal requires a light source to clearly and sharply display text and images. Therefore a typical LCD requires an accompanying backlight module, in order to ensure that images with sufficient, uniform brightness can be displayed.
Referring to FIG. 3, a typical backlight module includes a power supply circuit 10 and a plurality of lamps 11. Each lamp 11 includes a high voltage end and a low voltage end. The power supply circuit 10 is connected to the high voltage ends of the lamps 11, and provides alternating current voltages for the lamps 11. The low voltage ends of the lamps 11 are connected to ground. The lamps 11 are cold cathode fluorescent lamps (CCFLs).
FIG. 4 illustrates a waveform diagram of an ideal working voltage when a selected one of the lamps 11 is working. The ideal working voltage is a perfect sine waveform, that is, the ideal working voltage does not include any direct current component. The average value of the ideal working voltage is 0 volts.
FIG. 5-6 illustrate waveform diagrams of real working voltages when the selected lamp 11 is working. The real working voltages generally include direct current components. In FIG. 5, the real working voltage includes a direct current component Va greater than 0 volts. In FIG. 6, the real working voltage includes a direct current component Vb less than 0 volts.
The direct current component Va may force mercury ions in the lamp 11 toward the low voltage end. The direct current component Vb may force mercury ions in the lamp 11 toward the high voltage end. These may result in an accumulation of mercury ions at either end of the lamp 11. Thus, the distribution of mercury ions may become non-uniform. When this happens, the brightness of the lamps 11 is non-uniform, and a working lifetime of the lamp 11 is shortened. Accordingly, the performance of the backlight module is diminished.
What is needed, therefore, is a backlight module that can overcome the above-described deficiencies.